Purpose: Patients with colorectal cancer (CRC) often experience malnutrition and weight loss, largely resulting from reduced dietary intake. The aim of this study was to identify determinants of reduced dietary intake in order to facilitate early recognition of malnutrition and optimise nutritional treatment. Methods: Data from nutritionDay, an international 1-day survey investigating patient, disease and food profiles, were used. To identify determinants of dietary intake, defined as normal vs. reduced in the last week, univariate and multivariate logistic regressions were performed. Results: Of 1131 hospitalised CRC patients, 54% reported reduced dietary intake. Patient- and disease-related characteristics significantly associated with reduced dietary intake were female gender (odds ratio (OR) 1.38), cancer stage III (OR 1.52) or IV (OR 1.70) vs. I, performance status 2 (OR 1.56), 3 (OR 2.37) or 4 (OR 4.15) vs. 0, duration since hospital admission of ≥ 4 days (OR 4–7 days, 1.91; 8–21 days, 1.97; > 21 days, 1.92) vs. < 4 days, and unintentional weight loss (OR 2.56). Additionally, higher symptom scores of pain, weakness, depression, tiredness and lack of appetite were associated with reduced intake. Conclusions: Patient- and disease-related determinants for reduced dietary intake were being female, higher cancer stage, worse performance status, duration since hospital admission ≥ 4 days and unintentional weight loss. Furthermore, multiple symptoms were associated with a reduced dietary intake. Future trials should assess whether early recognition of patients at risk of malnutrition and the combination of treating symptoms and dietary advice result in improved intake and treatment-related outcomes.